1. Field of the Invention
The present invention relates to optical data recording systems. In particular, the present invention is an objective lens focus initialization system for an optical data recording system.
2. Description of the Prior Art.
Optical data recording technology has developed to the point where it is commonly found in many consumer electronic products. Optical video disks and optical compact audio disks have, for example, become very popular. This technology is also being adapted to high density optical data recording and storage systems. With continued advances in this technology it is believed that optical systems of this type will be able to compete in terms of performance and cost with the magnetic data storage systems currently in widespread use.
Optical data recording systems of the types referred to above include a record carrier, or disk, on which a single servo track is spirally positioned, or a plurality of servo tracks are concentrically positioned. A laser beam which is focused by an objective lens is used to write data onto, and to read data from, the servo tracks. Optical data recording systems also include a focus servo system for driving the objective lens about a focus axis, and maintaining the laser beam focused on a servo track. A tracking servo system is used to drive the objective lens along a tracking axis, and maintain the laser beam centered over a desired servo track.
To obtain the highest possible data density, individual servo tracks are positioned extremely close to one another on a recording surface of the record carrier. Minimum spacing is determined by various optical and physical properties of the system. Servo track spacings on the order of 1.6 .mu.m, for example, are common with current technology. Even though the servo tracks are placed extremely close together, their positioning with respect to the rotational axis of the record carrier is typically somewhat eccentric. In other words, the radial position of a given servo track from the rotational axis is not constant all the way around the record carrier. This eccentricity, or run-out, can range between 20 and 50 .mu.m per servo track revolution.
Upon the application of power to an optical data storage system, a focus initialization must be performed to bring the objective lens within focus capture range of the record carrier recording surface. Only after this initialization is performed is the focus servo system able to optically recognize the servo tracks and obtain information from them. Once the lens is positioned within focus capture range, the focus servo system is able to lock into focus, and the tracking servo system can lock onto a desired servo track. The focus initialization procedure is also performed when the focus servo system loses focus while in operation. Dirt or other flaws on the record carrier, or physical movement of the system, can, for example, cause focus loss.
In order to initialize focus, the focus servo system must have some criterion, or basis, to verify placement of the lens within focus capture range of the record carrier recording surface. One commonly used technique is described in U.S. Pat. No. 4,446,546. With this technique, the lens is simply moved toward the record carrier from the far end of its travel limit furthest from the record carrier. A focus error signal is monitored during this period of motion and focus capture indicated when the lens is at such a position that the focus error signal is at a point between its peaks.
Evaluating the focus error signal to verify focus capture does not always, however, produce optimum results. One common problem is the result of physical properties of the record carrier itself. Due to a high degree of sensitivity of the recording surface to dust, scratches and other physical imperfections, the recording surface is typically covered with a transparent protective layer. Notwithstanding its transparency, the protective layer will reflect radiation from the laser beam and produce stray or erroneous focus signals which known focus initialization systems can mistake for focus error signals. The focus servo system can use these stray signals to initialize focus on the protective layer, rather than the recording surface itself. In extreme cases, stray focus error signals can cause the objective lens to crash onto the protective layer. At best, the stray signals reduce overall performance of the focus initialization system.
There is clearly a continuing need for improved focus initialization systems. Performance requirements of high density optical data storage systems, in particular, require a focus initialization system which can quickly and accurately bring the objective lens within focus capture range of the record carrier recording surface. The focus initialization system should use criteria other than the focus error signal to verify focus capture. Such a system would be especially desirable if it were insensitive to stray reflections from the record carrier protective layer. Due to economic pressures in the marketplace, the focus initialization system must be inexpensive. It should, therefore, be capable of implementation with a minimum of hardware and software.